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# Category Archives: Current Electricity

## Internal resistance of a cell

A typical hand-held torch runs off two 1.5V cells yet has a lamp rated at 2.5V,0.5A.Explain how the potential difference across the lamp can actually be 2.5V as rated

## Resistors Combination – Numerical Problem

What is the effective resistance between P and Q in the diagram?

The Two arms have resistances 32 ohm and are in parallel, Their effective resistance is 1 ohms. This is in parallel with the middle arm. So the effective resistance is 8 ohms.

OR

The three arms have resistances 32, 1 and 32 respectively and are in parallel. Therefore, taking their reciprocals and adding will give the answer.

## Current Electricity – Drift Velocity based numerical

Two different wires X and Y of same diameter but different materials are joined together in series     across a battery . If the number density of electrons in X is  twice that in Y . Find ratio of drift velocity of electrons in two wires .

In series combination, the current is the same.

I = nAev

=> n1v1 = n2 v2

=> v1:v2 =n2:n1

## Questions from Alternating current

1.      Derive an expression for the average power in an ac circuit.
2.      Distinguish between resistance, reactance and impedance.
3.      Draw the phasor diagram showing voltage and current in LCR series circuit and derive an expression for the impedance

4.      Deduce the phase relationship between current and voltage in an ac circuit containing inductor only
5.      A series LCR circuit with L = 0.12 H, C = 4.8 x 10 7 f and R = 23 ohms is connected to a variable frequency supply.  At What frequency is the current maximum?
9.      A sinusoidal voltage V=200 sin 314t is applied to a resistor of 10 ohms resistance. Calculate (i) rms value of the voltage, (ii) rms value of the current and (iii) power dissipated as heat in watts.

(Posted by Sayantika Nath)

## Doubts and Questions from AC

what is phase difference?
what is resonance?
and resonance frequency ?

From: jogesh
Date: Thu, Dec 20, 2012 at 4:16 PM
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## Transformers

Please explain what is transformer and tell what is the reason behind the working of transformer that it reduces potenial when current is high and increases potential when current is low?

Explain widely and deeply with concepts.

Transformer is a device based on mutual inductance and is used to change the voltage of alternating (or varying) current. There are two types of transformers – stepup and stepdown.

A step-up transformer increases the voltage as the step-down transformer decreases the ac voltage.

When a transformer changes the alternating voltage, it is not adding any power, it only transforms the voltage.

As power is the product of voltage and current, as voltage increases, the current decreases. (and vice versa)

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